In general, a floating support has anchor means for remaining in position in spite of the effects of currents, of winds, and of swell. In general, it also has means for drilling, for storing, and for processing petroleum oil, and means for offloading to offload tankers that call at regular intervals to offload the production. Such floating supports are known as “Floating Production, Storage & Offloading” units, referred to below as “FPSO units”, or, when the floating supports are also used to perform drilling operations on a well that is deflected in the depth of the water, such supports are known as Floating Drilling & Production Units” or “FDPUs”.
A riser of the invention can be a crude-oil or gas “production riser”, or a water injection riser, forming a link with an undersea pipe lying on the sea bottom, or indeed a “drilling riser” forming link between the floating support and a wellhead located on the sea bottom.
In FPSO units in which, in general, a multiplicity of lines are installed, the links implemented are either of the hybrid tower bottom-to-surface type, or of the catenary type.
When the bottom-to-surface link pipe is of the catenary type, it forms a direct link between a floating support and a contact point on the sea bottom that is offset relative to the axis of said support, with said pipe taking up a “catenary” configuration under its own weight, thereby forming a curve whose radius of curvature decreases going from the surface to the contact point on the sea bottom, and the axis of said pipe forms an angle α with the vertical of value that varies, in general, from 10-to-20 degrees at the floating support to, ideally, 90 degrees at the sea bottom, corresponding to an ideal position that is substantially tangential to the horizontal, as explained below.
Catenary-type links are generally formed by means of flexible pipes, but they are of extremely high cost due to the complex structure of the pipe.
Therefore, substantially vertical risers have been developed in order to have the flexible link of catenary configuration close to the surface, in the vicinity of the floating support, thereby making it possible to minimize the length of said flexible pipe, and to minimize the forces that are applied to it, thereby also considerably reducing its cost.
In addition, for petroleum oil production, since the crude oil is conveyed over distances that are very long, several kilometers, it is desirable to provide an extreme level of insulation for them, firstly so as to minimize the increase in viscosity that reduces the hourly output of the well, and secondly so as to avoid the flow becoming blocked by paraffin being deposited or by hydrates forming whenever the temperature goes down to a value approximately in the range 30° C. to 40° C. Such phenomena are even more critical when the sea bottom temperature is about 4° C. and the crude oil is of the paraffinic type, as applies, in particular, off West Africa.
Whenever the depth of water reaches and exceeds in the range 800 meters (m) to 1000 m, it becomes possible to form said bottom-to-surface link by means of a rigid pipe having a thick wall because, since the length of the pipe is considerable, its flexibility makes it possible to obtain a catenary configuration that is satisfactory while remaining within acceptable stress limits.
Such rigid risers made of strong materials of large thickness and disposed in catenary configurations are commonly known as “Steel Catenary Risers” and are referred to in the present description as “SCRs”, regardless of whether they are made of steel or of some other material such as a composite material.
Such catenary risers or “SCRs” are much simpler to make than flexible pipes, and are thus less expensive.
The geometrical curve formed by a pipe of uniform weight in suspension subjected to gravity, and referred to as a “catenary” is a mathematical function of the hyperbolic cosine type (cos hx=(ex+e−x)/2, relating the abscissa and the ordinate of any point of the curve using the following formulae:y=R0(cos h(x/R0)−1)R=R0(y/R0+1)2 
where:
x represents the distance in the horizontal direction between said contact point and a point M of the curve;
y represents the altitude of the point M (x and y are thus the abscissa and the ordinate of a point M of the curve relative to a rectangular frame of reference whose origin is at said contact point);
R0 represents the radius of curvature at said contact point, i.e. at the point of horizontal tangency; and
R represents the radius of curvature at point M (x,y).
Thus, the curvature varies along the catenary from the surface, where its radius has a maximum value Rmax, to the contact point, where its radius has a minimum value Rmin (or R0 in the above formula). Under the effect of waves, of wind, and of current, the surface support moves laterally and vertically, thereby lifting the catenary-shaped pipe off the sea bottom and putting it back down thereon.
Thus, the pipe has a radius of curvature that is at its maximum at the top of the catenary, i.e. at the point where it is suspended from the FPSO unit. Said maximum radius of curvature is, in general, at least 1500 m, and in particular lies in the range 1500 m to 5000 m, and said radius of curvature decreases going from said top down to the point of contact with the bottom. At this location, the radius of curvature is at its minimum in the portion that is in suspension. However, in the adjacent portion lying on the sea bottom, and since said pipe is ideally in a straight line, the radius of curvature is theoretically infinite. In fact, said radius is not infinite, because some residual curvature remains, but it is extremely high.
Thus, as the floating support moves on the surface, the contact point moves forwards and backwards and, in the zone in which the pipe is lifted off or put back down on the bottom, the radius of curvature goes successively from a minimum value Rmin to an extremely high value, or even to infinity for an ideal configuration in which the undersea pipe lies on the sea bottom substantially in a straight line.
This alternating bending gives rise to fatigue phenomena that are concentrated throughout the catenary foot zone and the lifespan of such pipes is greatly reduced and is, in general incompatible with the lifespans desired for bottom-to-surface links, i.e. in the range 20 years to 25 years, or even longer.
In addition, during such alternating movements of the contact point, it is observed that the stiffness of the pipe, associated with the above-mentioned residual curvature, acts, over time, to cause a furrow to be dug over the entire length lifted off and then put back down again, and to cause a transition zone to be formed in which there exists a point of inflection at which the radius of curvature, which is at its minimum at the foot of the centenary, then changes sign in said transition zone, and increases to reach, ultimately, an infinite value in the portion of undersea pipe that is lying in a straight line on the sea bottom.
In poorly-consolidated soil of the type commonly encountered at great depths, such repeated movements over long periods form a furrow that is particularly deep and the effect of this is to change the curvature of the catenary and, if the phenomenon becomes larger, to give rise to risks of the pipes being damaged, either in the undersea pipes lying on the sea bottom, or in the SCRs linking said undersea pipes lying on the sea bottom to the surface.
The most critical portion of the catenary is thus situated in its portion close to the contact point, and, in fact, most of the forces in this low portion of the catenary are generated by the movements of the floating support itself and by the excitations that occur in the high portion of the catenary that is subjected to current and to swell, all of these excitations then propagating mechanically all the way along the pipe to the foot of the catenary.
In this catenary foot zone, pipes are made of steels that are selected to withstand fatigue throughout the life of an installation, but the welds between pipe elements constitute weak spots when said pipe conveys either water, or fluids including water, and more particularly saltwater. In the presence of water, said welds are subjected to corrosion phenomena that, over time, and under variable bending stresses, give rise to incipient cracks that lead to the ruin of said pipe.
In order to mitigate that problem, welds are formed between pipe elements by means of a stainless steel or of a corrosion-resistant alloy. Anti-corrosion alloys are well known to the person skilled in the art. They are mainly alloys based on nickel, in particular of the Inconel type, and preferably of a particular grade, in particular Inconel 625 or 825, which Inconels also offer excellent fatigue strength due to their high elastic limits and thus make it possible to achieve lifespans lying in the range 20 years to 30 years.
However, in order to enable the welding to be performed under good conditions, it is necessary to clad the insides of the two pipe elements that are to be welded together with the same stainless steel or corrosion-resistant alloy over a few centimeters (cm) at the ends of those pipe elements, so that the weld penetration pass that is to constitute the future wall in contact with the fluid is of the same metal as the filler metal of the weld, in particular of Inconel. That cladding of stainless steel or of anti-corrosion alloy, in particular of the Inconel type, is formed by a costly electric arc method known as “cladding” that is generally implemented using a Tungsten Inert Gas (TIG) welding method associated with a filler rod of stainless steel or of corrosion-resistant alloy.
That portion of the inside surface of the pipe that is clad with stainless steel or with anti-corrosion alloy of the Inconel type gives rise to a major problem for the weld inspection that is performed from the outside using scanning acoustic means to obtain very precise mapping of defects in the weld. The TIG method is highly localized and high-energy, and the weld pool of molten parent steel and of molten alloy gives rise to undulations at the interface between the two metals, thereby making any inspection by ultrasound almost impossible. During ultrasound inspection, such undulations at the interface being the two metals generate echoes that make it almost impossible to use the results in practice, and thus to show the locations of the defects, and, as a result, prevent the quality of the weld between the two pipe elements from being determined.
The portion of the inside surface of the pipe that is clad with stainless steel or with anti-corrosion alloy of the Inconel type gives rise to another major problem at the inside wall of the pipe that is in contact with the fluid, and more particularly in the transition zone in which the metal of the wall changes, i.e. where it changes between the conventional carbon steel of the main portion of the pipe element and the stainless steel or anti-corrosion alloy of the Inconel type. In said transition zone, and in the presence of water, a phenomenon occurs whereby the ordinary steel in contact with the water is subjected to electrochemical corrosion, i.e. to galvanic corrosion or “dissimilar-metal corrosion”, that locally generates corrosion spots in the steel in contact with the water in the vicinity of the stainless steel or of the anti-corrosion alloy, and that can give rise to cracking under repeated bending stresses, and the resulting cracks are totally unacceptable if the pipe is to offer good fatigue endurance.
That type of anti-corrosion weld is thus not only difficult to achieve, in particular when the pipe is assembled by welding on a laying ship at sea, but it is also unsatisfactory because it cannot be inspected reliably by conventional ultrasound testing, and it gives rise to phenomena of corrosion of the steel of the inside surface of the pipe in the proximity of the weld.
Methods are also known for protecting the steel inside surface of the pipe from corrosion by implementing a continuous lining by means of a flexible cladding or “liner” made of thermoplastic materials. Unfortunately, such linings are also costly and complex to implement because they require, in particular, anchoring at the ends by means of ferrules or sleeves so that the lining stays in place while the pipe is conveying a fluid at high pressure. Those sleeves are also complex and costly to put in place. In addition, the ends of the strings or elements of pipe to be assembled together by welding must not be pre-clad with said lining because such a lining cannot withstand the high temperatures implemented during welding operations. Finally, such continuous internal linings require lining thicknesses lying at least in the range 10 millimeters (mm) to 15 mm, which represents a very substantial decrease in the inside diameter of the pipe, and thus gives rise to additional head loss of the fluid conveyed inside the pipe in operation.